Algebric Sets

Algebra Level 4

{ 1 , 2 } , { 4 , 5 , 6 } , { 9 , 10 , 11 , 12 } , { 16 , 17 , 18 , 19 , 20 } , \{1,2\} , \{ 4,5,6 \} , \{9,10,11,12\} , \{16,17,18,19,20\} , \ldots

The first n n positive integers are partitioned into sets as described above.

Let S n S_{n} be the sum of the elements of the set at n th n^{\text{th}} position. Find the value of 3 S 50 S 51 S 50 . \sqrt{3} \dfrac{S_{50}}{\sqrt{S_{51}-S_{50}}}.


The answer is 2525.

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3 solutions

Danny He
Nov 10, 2014

For the n t h n^{th} set we see that it contains the numbers from n 2 n^2 to n 2 + n n^2 + n

Therefore the sum of the numbers in the n t h n^{th} set is:

n 2 + ( n 2 + 1 ) + ( n 2 + 2 ) + . . . + ( n 2 + n ) n^2 + \left(n^2+1\right) + \left(n^2 + 2 \right) + ... + \left(n^2 + n \right)

= n 2 ( n + 1 ) + 1 2 n ( n + 1 ) = n^2 \left(n+1\right) + \frac{1}{2}n\left(n+1\right)

= 1 2 ( n + 1 ) ( 2 n 2 + n ) = 1 2 n ( n + 1 ) ( 2 n + 1 ) = \frac{1}{2}\left(n+1\right)\left(2n^2+n\right)=\frac{1}{2}n\left(n+1\right)\left(2n+1\right)

Therefore, we see that ( 3 ) S 50 S 51 S 50 \sqrt(3)\frac{S_{50}}{\sqrt{S_{51}-S_{50}}}

= ( 3 ) 1 2 ( 50 ) ( 51 ) ( 101 ) 1 2 ( 51 ) ( 52 ) ( 103 ) 1 2 ( 50 ) ( 51 ) ( 101 ) = \sqrt(3)\frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{\sqrt{\frac{1}{2}\left(51\right)\left(52\right)\left(103\right) - \frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}}

= 3 1 2 ( 50 ) ( 51 ) ( 101 ) 1 2 ( 51 ) ( 52 103 50 101 ) = \sqrt{3}\frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{\sqrt{\frac{1}{2}\left(51\right)\left(52*103 - 50*101\right)}}

= 3 1 2 ( 50 ) ( 51 ) ( 101 ) 1 2 ( 51 ) ( 5356 5050 ) = \sqrt{3}\frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{\sqrt{\frac{1}{2}\left(51\right)\left(5356-5050\right)}}

= 3 1 2 ( 50 ) ( 51 ) ( 101 ) 1 2 ( 51 ) ( 306 ) = \sqrt{3}\frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{\sqrt{\frac{1}{2}\left(51\right)\left(306\right)}}

= 3 1 2 ( 50 ) ( 51 ) ( 101 ) ( 51 ) ( 51 ) ( 3 ) = \sqrt{3}\frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{\sqrt{\left(51\right)\left(51\right)\left(3\right)}}

= 1 2 ( 50 ) ( 51 ) ( 101 ) 51 = \frac{\frac{1}{2}\left(50\right)\left(51\right)\left(101\right)}{51}

= 25 101 = 2525 = 25*101 = 2525

same method as of me ! :)

Nihar Mahajan - 6 years, 7 months ago

Same as @Parth Lohomi

U Z - 6 years, 7 months ago

I instead used the same method but made a mistake in the calculation

Aakash Khandelwal - 6 years, 6 months ago
Parth Lohomi
Nov 10, 2014

First term of each set is square of the number of the set so the first term of 50th set will be 2500

And last term of the nth set is given by n(n+1) so the last term of the 50th set will be 2550

All the terms will be in A P AP

thus the sum of the terms of the 50th set will be 128775

Similarly doing in the 51th set the first term will be 5 1 2 51^{2} =2601

And the last term 2654

And all these are in AP THUS THE SUM OF ALL THE terms will be 136578

So we have got S(50) and S(51) on putting in the question we get s51-s50=7803

7803 \sqrt{7803} =51 3 \sqrt{3} ----------(S51-S50)

So final answer is

128775 51 \frac{128775}{51} 3 \sqrt{3} * 3 \sqrt{3} = 2525 \boxed{2525}

This is our answer

exactly my solution. :D

Aritra Jana - 6 years, 7 months ago
Srikanth Kummara
Dec 6, 2014

Observe sets.....each set has one element more than its position and highest value is position * no of elements.... So s50 has 51 elements starting from 2500 to 2550 and s51 has elements starting from 2601 to 2652(51*52)....find both sums using arithmetic progression and substitute. And is 2525.

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